1. Field of the Invention
The invention relates generally to a direct access storage device (DASD) of the type utilizing partial-response maximum-likelihood (PRML) detection, and more particularly to a method and apparatus for thermal asperity compensation for PRML data detection.
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. A slider supports one or more magnetic heads. The slider is lightly biased to cause the heads to move toward the recording surface when the disk is stationary; but as the disk is brought up to operating speed, an air bearing is generated which moves each slider and hence the heads away from the recording surface toward a preselected flying height. Achievement of a higher data density on magnetic disks has imposed increasingly narrow transducing gaps.
A magneto-resistive (MR) transducing head exhibits a change in resistance when in the presence of a changing magnetic field. This resistance change is transformed into a voltage signal by passing a constant current through the MR element. The value of DC voltage, for a given head, is the product of the constant bias current and the total resistance between the head lead terminals. The temperature coefficient of resistivity of the MR material is 0.02%/degree C.
A phenomena, termed thermal asperities, can locally increase the strip temperature by more than 100 C. degrees. The cause of this temperature rise is a mechanical collision of the portion of the head containing the MR stripe with a protrusion on the disk surface. Since the change in resistance, as a function of the magnetic field due to read signal in the media, is less than 1% of the total MR stripe resistance, the signal step that is added to the read signal when a thermal asperity is encountered can be greater than twice the base-to-peak read signal. An increase in the temperature of the stripe of 100 C. degrees would cause a resistance change and a voltage change of 2%. When the protrusion on the disk is persistent and the head continues to strike it each revolution, then the data that is being modulated by the resultant thermally induced signal transient will be unreadable without a sufficient error correction code.
Known arrangements for minimizing the effect of thermal asperities on the read data utilize a separate circuit or asperity reduction circuit (ARC) module for additive disturbance transient suppression for data channels. Disadvantages of the know arrangements include the hardware required and the corresponding electronics cost and the required error burst length for a given thermal transient amplitude. The relatively long error site limits its applications. For example, because much more redundancy in the error correcting code or compensation (ECC) is required than is tolerable for small fix-blocked formatted disk drives.